PROJECT SUMMARY/ABSTRACT Adolescents with substance use disorders (SUD) are at high risk for significant deleterious outcomes. Although several evidence-based practices for adolescent SUD exist, they yield only small to medium effects that rapidly diminish (30-70% 6-month relapse rates). A promising approach for determining how to enhance treatment is experimental mediation research. In contrast to traditional correlational mediation approaches, experimental mediation permits causal inference and is comprised of key steps: (A) Identify the putative mediating variable for a treatment. (B) Enhance the treatment to target that mediator more intensely. (C) Randomize youth to conditions, with the standard and enhanced versions of the treatment targeting different ?levels? of the same mediator. (D) Measure the mediator and outcomes longitudinally. (E) Perform modern mediation analyses, coupled with analyses evaluating causal inference, to determine if changes in the mediator are responsible for changes in outcomes. This experimental test of mediation, focused on causality, facilitates rapid improvement of treatments by specifying change mechanisms to target in order to improve outcomes. These steps will be followed to elucidate the mediating processes in treatment for adolescent SUD, with the ultimate goal of enhancing the strength and durability of SUD treatments. The three most common putative mediating variables in adolescent SUD treatments are parent management, behavioral regulation, and peer relations. For this study, parent management was chosen as the target because it has evidenced the most potential for yielding generalizable change in youth outcomes and also has been shown to indirectly improve youths? behavioral regulation and peer relations. Of existing treatments for adolescent SUD, family-based Contingency Management (CM) was chosen as the treatment to enhance because it is highly amenable to an augmented focus on parenting, is less complex relative to other SUD treatments, and has amassed considerable support in terms of efficacy and dissemination potential. Thus, following experimental mediation steps, youth with SUD will be randomized to receive either standard CM or enhanced CM (i.e., CM+) that targets parenting more intensely. Repeated assessments for 12 months and longitudinal analyses will allow testing of mediating processes. We will examine whether parent management skills mediate the effect of treatment on youth substance use and behavior problems (Aim 1). In addition, we will determine whether parent management skills mediate the effect of treatment on youth behavioral regulation and deviant peer relations (Aim 2). Finally, we will test whether behavioral regulation and deviant peer relations mediate the effect of parent management on youth substance use and behavior problems (Aim 3). Findings could have broad impact across multiple adolescent SUD treatments.